Carburized Parts subjected to surface-hardening treatment by carburization have been used as gear wheels, bearings and like parts in automotive transmissions or differential devices. On such carburized parts, various studies of fatigue strength have been carried out from both the viewpoint of a fatigue failure caused by a load imposed repeatedly at about 105 or more times (high cycle fatigue) and the viewpoint of a fatigue failure caused by a load imposed repeatedly at about 103 or less times (low cycle fatigue).
For example, Patent Document 1 has disclosed that adjustments to compositional proportions of Si, Mn and Cr in a steel for carburized part can improve high cycle fatigue strength of the carburized part subjected to surface-hardening treatment by carburization. In the case where a steel for carburized part, such as SCM21, is subjected to a carburizing treatment, and subsequently a heat treatment, a poorly hardened portion (a carburization anomaly layer) develops in the vicinity of the carburized steel surface, and causes degradation in high cycle fatigue strength in particular. With such a carburization anomaly layer formed by internal oxidation of Si, Mn and Cr, Patent Document 1 has concluded that adjustments to the compositional proportions of those elements in the steel allow improvement in the high cycle fatigue strength. More specifically, Patent Document 1 has disclosed that, in the steel which has, in terms of % by mass, a C content of 0.05% to 0.50%, an Si content of 0.05% or lower, an Mn content of 5% or lower and a Cr content of 5% or lower and can further contain other elements, such as Ni, Mo, Ti, V, Nb, Al and B, in proportions lower than individually specified values, contents of the elements Si, Mn and Cr having an effect on internal oxidation are required to satisfy the following relation;10[Si]+0.1([Mn]+[Cr])≦1.00wherein [M] represents a content of element M in terms of % by mass.
On the other hand, Patent Document 2, for example, has disclosed that the low cycle fatigue strength of a carburized part subjected to surface-hardening treatment by carburization can be improved by adjusting the composition of a steel for carburized part to be relatively high in Cr and Mn contents. Further, Patent Document 2 stated that not only toughness can be enhanced by limiting the C content to a lower value yet ensuring hardness in the vicinity of the carburized surface but also the low cycle fatigue strength can be improved by controlling the hardness difference between the vicinity of the carburized surface and the core portion so as to fall within a specified range. More specifically, Patent Document 2 has disclosed that, in the steel which has, in teems of % by mass, a C content of 0.05% to 0.20%, an Si content of 0.7% or lower, an Mn content of 1.41% to 2.0% and a Cr content of 1.0% to 2.0% and can further contain other elements, such as Ni, Mo, Ti, Nb, Al and B, in proportions lower than individually specified values, the C content in the carburized layer at the carburized surface is adjusted to fall within the range of 0.4% to 0.75% by mass and the hardness difference between the vicinity of the carburized surface and the core portion is controlled to the 200- to 400-Hv range.
By the way, studies on fatigue failure caused by a load imposed repeatedly at about 103 to about 105 times, or equivalently, medium cycle fatigue failure, has also come to be made in recent years. In general the medium cycle fatigue strength can be enhanced by carrying out improvements in both the low cycle fatigue strength and the high cycle fatigue strength.
Patent Document 3, for example, has stated that the medium cycle fatigue strength can be enhanced by adjusting the depth of a carburization anomaly layer formed after a carburization-based surface hardening treatment to 15 μm or less and reducing variations in the depth while controlling the contents of Si, Mo and B in a steel for carburized part. Therein, the low cycle fatigue strength is improved by enhancing the strength of grain boundaries in the carburized layer, and at the same time, occurrence of fatigue cracking is reduced by controlling the depth of a carburization anomaly layer and variations therein, which conduces to an enhancement of the high cycle fatigue strength. Thus Patent Document 3 has concluded that the medium cycle fatigue strength can also be improved.
In addition, Patent Document 4, for example, has disclosed that the depth of a carburization anomaly layer formed after a carburization-based surface hardening treatment can be adjusted to have a specified value or lower by controlling the contents of Si, Mn and Cr in a steel for carburized part. More specifically, Patent Document 4 has disclosed that, in the steel which has, in terms of % by mass, a C content of 0.15% to 0.25%, an Si content of 0.1% or lower, an Mn content of 0.2% to 0.8% and a Cr content of 0.2% to 0.8% and can further contain other elements, such as Ni, Mo, Ti, Nb, Al and B, in proportions lower than individually specified values, the depth of a carburization anomaly layer can be reduced to 6 μm or less by adjusting the C content in the carburized layer to fall within a range of 0.7% to 0.9%, the grain size in the carburized layer to be #9 or above and the contents of elements Si, Mn and Cr to satisfy the following relation;10[Si]+[Mn]+[Cr]≦2.0wherein [M] represents a content of element M of the carburized layer in terms of % by mass.
[Patent Document 1] JP-A-S51-90918
[Patent Document 2] JP-A-2008-248284
[Patent Document 3] JP-A-2010-150592
[Patent Document 4] JP-A-H6-306572